The present invention relates to a scorotron charging apparatus for an electrostatic copying machine or the like.
In an electrostatic copying machine a photoconductive drum or the like is electrostatically charged by a corona charging unit and radiated with a light image of an original document to form an electrostatic image through localized photoconduction. Toner is applied to the drum to develop the electrostatic image into a toner image which is transferred and fixed to a copy sheet to provide a permanent reproduction of the original document.
It is desirable to ensure that the initial electrostatic charge applied to the drum has a predetermined value under various conditions of temperature, etc., and it is especially necessary to prevent the drum from becoming overcharged. If the charge has too high or low a potential, the density of the copy will be too high or too low respectively. If the charge potential exceeds the breakdown voltage of the photoconductive coating on the drum, the photoconductive coating will become permanently damaged.
To provide this function, corona chargers known in the art as "scorotron" chargers have been developed. Typical examples of such chargers are disclosed in U.S. Pat. Nos. 2,777,957 and 2,778,946 and comprise a corona charging electrode. A plurality of wires are disposed between the electrode and the surface to be charged. A high voltage is applied to the electrode. A low voltage which is slightly lower than the desired predetermined potential to be formed on the drum is applied to the wires. The wires prevent the drum surface potential from exceeding a certain value.
When the charge on the drum surface is below the potential on the wires, ion current from the electrode flows to both the wires and the drum surface. The ion current flow to the drum surface increases the electrostatic potential thereon, or in other words charges the surface. However, as the surface potential somewhat exceeds the potential on the wires, a reverse electric field is produced between the surface and the wires which repels the ions back toward the electrode. When the surface potential is sufficiently greater than the potential on the wires, an equilibrium condition will be created in which there is no further ion current from the electrode to the surface. In this case, all ion current flow will be from the electrode to the wires.
In actual practice, however, it has been discovered that ion current to the surface does not completely cease even when the predetermined potential is reached, and a certain amount of leakage current enables further charging of the surface. A prior art expedient has been proposed to reduce this leakage current to a negligible value. The expedient consists of decreasing the spacing between the wires in the direction of movement of the surface. Thus, at the downstream end, the spacing between the wires is smaller than at the upstream end. This has the effect of progressively choking off the ion current to the surface and reducing it near zero at the downstream end of the charging apparatus.
Although this method is reasonably effective in eliminating the leakage current, it has been determined in actual practice that the spacing between the wires must be reduced to such an extent that the width of the electrode and the power supply thereto must be increased to a disproportionate extent to overcome the increased shielding effect of the wires and allow the surface to be charged to the required potential. These conflicting requirements dictate that the charging apparatus must be overly large in size for practical application and be supplied with an excessive voltage which is detrimental to economy and safety.